Thunderstone Search Appliance Manual

If multiple custom primer URLs are being used, you can control which
ones are used for which Base URLs via Base URL MM Query.

By default, primer URLs are only used on Base URLs that have a matching
protocol and hostname. If Base URL MM Query is non-empty,
then this Metamorph query will be run against the Base URL being
walked. The associated primer URL will only be fetched if it
matches.